Granville F. Foster

“ In the case supposed, since A is one-quarter of the mass of F, a given force must necessarily move A over four times the space and with four times the velocity that it is able to move F, and when A and F meet the momenta of A and F will be respectively equal.
The truth that two bodies mutually and equally attract each other is also abundantly proved in astronomy. Take the case of the earth and moon. The earth by its attraction compels the moon to make around it as a centre her monthly revolution ”

“ Calling the force which A exerts on B, or B exerts on A, one, then will the sum of the forces which B, C, D, and E exert on A be equal to four, but the sum of A's attractions for B, C, D, and E, will also be equal to four, since A's attraction for B is in no way either increased or diminished by the fact that at the same time it also exerts an attraction on C, D, or E. Now, let B, C, D and E, be united into one mass, F, and it will be readily perceived that the truth of the foregoing statements cannot thereby be affected. ”

“ The same thing is true of the sun, which is obliged by the combined forces of the planets to revolve around the centre of gravity of the solar system, and on making the necessary calculations we find that the force exerted on the planets by the sun just equals the force exerted by the planets on the sun. ”

“ If one of the masses contain m units of mass, and the other one unit, the force will be m times as great as though they were both units of mass; but if the second body contain n units of mass, the attraction will be n times as great as before ”

“ Now, it will require but little reflection to perceive that to move the earth in a circle with a radius of a little more than 2,000 miles, and the moon in a circle with a radius of nearly 240,000 miles, would require equal forces. ”